The present invention relates to an improved drive arrangement for a unit of a printing press.
A known printing press drive arrangement is disclosed in U.S. Pat. No. 3,724,368 and includes two gears connected to each blanket cylinder. One set of blanket cylinder gears drive the blanket cylinders. Another set of blanket cylinder gears drive the plate cylinders through a drive train which includes plate cylinder gears and harmonic drive units. The harmonic drive units contain flexible splines having external gear teeth which cooperate with internal gear teeth to rotate the plate cylinders relative to the plate cylinder gears. To compensate for the effect of the gears in the harmonic drive units, the number of teeth on the set of blanket cylinder gears which drive the plate cylinder gears is less than the number required to rotate the plate cylinders at the same surface speed as the blanket cylinders. This enables the output from the harmonic drive units to rotate the plate cylinders at a surface speed which is equal to the surface speed of the blanket cylinders.
To compensate for the action of the gearing in the harmonic drive units, the set of blanket cylinder gears which drive the plate cylinder gears in the known drive arrangement have 119 teeth. The plate cylinder gears have 60 teeth. Since the blanket cylinder has a diameter which is twice as great as the diameter of the plate cylinder, the blanket cylinder gears would normally have twice as many teeth as the plate cylinder gears, that is 120 teeth. By providing the blanket cylinder gears which drive the plate cylinder gears with a relatively small number of teeth, the output from the gearing in the harmonic drive unit results in the plate cylinder being driven at the same surface speed as the blanket cylinder.
When standard gears having a relatively small number of teeth are substituted for standard gears having a larger number of teeth and the same pitch, the substituted gears will have pitch diameters which are smaller than normal for the distance between the centers about which the gears rotate. Thus in the known drive system, the 119 tooth blanket gears which drive the plate cylinder gears have a pitch diameter which is less than the pitch diameter of a 120 tooth blanket cylinder gear of the same pitch.
The relatively small pitch diameter of the standard blanket cylinder gears utilized to drive the plate cylinder gears in the known drive system results in a relatively large amount of backlash between the plate and blanket cylinder gears and adversely effects the driving relationship between the gears. In addition to adversely effecting the driving relationship between the gears when the known printing press is being utilized for printing operations, the relatively small pitch diameters of the standard blanket cylinder gears results in a relatively small overlap between the teeth of the blanket cylinder gears when the blanket cylinders are thrown off. Therefore, the blanket cylinder gears may not reliably carry the drive forces for driving the blanket cylinder gears when thrown off. To compensate for this, the known printing press drive arrangement disclosed in U.S. Pat. No. 3,724,368 utilizes two gears in association with each blanket cylinder. Of course, the necessity of providing two gears with each blanket cylinder increases the cost of constructing the printing press.